This invention relates to a positioning mechanism for beam projection lamps and, in particular, relates to a hydraulically controlled gimbal mount structure for precisely positioning the projected beam.
In the prior art, a wide variety of beam-positioning or movable lamp-mounting means have been proposed. A difficulty with all known prior art systems is the impreciseness with which the beam is positioned. While for application purposes this may not be critical, precise beam positioning is very important in testing the lamps, particularly in the automotive area where lamps must comply with specified beam patterns.
A second difficulty with prior art systems is that beam-positioning accuracy, if obtained, is gotten only after a considerable amount of time has been expended adjusting the position of the beam. The time required for positioning the beam and the time required for the photometry itself combine to make testing beam projection lamps a time-consuming and tedious process. Typically, the measurements are made manually by positioning the lamp, monitoring the voltage across the lamp, and recording the current from a photocell.
Depending upon the particular test being carried out, this procedure can take as long as one and one-half days for a single lamp. For example, the beam pattern of an automotive headlamp is given by what is known as an isocandle curve, in which a plurality of closed curves indicate the coordinates in space where the illumination is the same. To obtain these curves, the above procedure is repeated for each of a large number of points and the closed curves drawn to include the points.
Another test, involving fewer points but generally requiring one to two hours per lamp to complete, is the test for compliance with the S.A.E. (Society of Automotive Engineers) standards for headlamps. In this test, the beam pattern is checked at 14 locations for maximum or minimum illumination.
In general, it is desired to reduce the time required for these tests and to provide more precise information about the beam pattern. To this end, various systems have been proposed in the past. One such system utilizes a lamp-positioning mechanism controlled by photocell detection circuitry. The circuitry monitors the current generated by a photocell and positions the lamp so that the photocell receives the same level of illumination as the beam is moved. Electric signals indicative of position control the position of a pen in a plotting mechanism to draw the isocandle curves. While faster than the manual procedure, the system is difficult to control. Further, the S.A.E. points are not evaluated except by chance, i.e., the system uses a test variable (position) as a control, which is not preferred.
While the positioning mechanism of the present invention could be used in such a test system, a preferred system utilizes the positioning mechanism of the present invention with the photometry described in the above-noted concurrently filed application, wherein both are under the control of the user and/or a computer.
A variety of positioning mechanisms have been proposed in the prior art, some using gears, others using cables or rigid mechanical linkages to impart motion to the lamp. Typically, these systems read position from the actuating mechanisms for the two axes of rotation rather than from the actuated mechanism. This reduces accuracy since the position reading may not correspond to the actual position due to slack in the mechanism, e.g., backlash in the gears.
One reason separate actuation and readout have been avoided is the high moment of inertia such a system would have, requiring large drive forces and exhibiting momentum problems. Thus, for example, one prior art system uses a "Y" shaped yoke, i.e., a half ring supported at the middle, to impart rotation in the horizontal plane, and a full ring inside, supported across its diameter at the ends of the outer half ring. Since motion in the horizontal plane is defined by the yoke, i.e., the entire support must rotate, the moment of inertia in the horizontal plane is very high. This is a distinct disadvantage in headlamp photometry since the beam pattern being evaluated is extremely rectangular, with the long dimension horizontal. Thus, a large number of horizontal scans are required in which the drive system must act against the high moment of inertia and still provide accurate positioning. (Vertical scanning is not feasible since the number of scans, more particularly the number of turnarounds, becomes very high, greatly increasing the time it takes to complete the isocandle curve.) As with other systems of the prior art, the position data is taken from the actuator rather than from the yoke itself.
Hydraulic control has generally not been considered for the actuating mechanism due to the nature of hydraulic motors available in the prior art. These motors have high "break-away" force, which has been found to result from the type of seal used in the motor. Stated another way, the seals have high static and low dynamic friction, typically requiring 30-40 pounds per square inch (psi) pressure for break-away in small motors. The result is that it is difficult or impossible to precisely control shaft rotation with these motors. This, combined with a separate drive and position readout, would be generally considered to produce a totally unworkable system.